Interpretive Summary: Infiltration and runoff rates on rangelands can vary considerably due to a number of interrelated factors including soil and plant characteristics, slope gradient and length, microtopography, and rainfall rates and duration. This study addresses the effects of runoff flow depth and microtopography on infiltration rates in a shrub dominated ecosystem. The concept is that the infiltration rates under the shrubs are higher than in the areas between the shrubs. Because the shrubs occur on mounds, this means that as the runoff flow depth increases, the area covered by water includes topographically higher areas that have higher infiltration rates thus increasing the area averaged infiltration. A model was developed that combines the above concept with partial contributing area under variable rainfall intensity and the widely used Green-Ampt infiltration equation. The performance of the new model was compared with that of a conventional Green-Ampt equation using plot and small watershed rainfall runoff data from the USDA Walnut Gulch Experimental Watershed in south eastern Arizona. The new model reproduced hydrographs better for the plot data while both models did equally as well for the small watershed data. The proposed model offers some practical and theoretical advancement towards a less scale dependent way of modelling runoff at the hillslope scale.

Technical Abstract:
Observed scale effects of runoff and erosion on hillslopes and small watersheds pose one of the most intriguing challenges to modellers, because it results from complex interactions of time-dependent rainfall input with runoff, infiltration and macro- and microtopographic structures. A little studied aspect of scale effects is the concept of water depth-dependent infiltration. For agricultural systems it has been shown that low infiltrability areas are associated with low lying sedimentary crusts within the tillage microtopography, while high conductivity areas are associated with the slopes and crests of aggregates and ridges. Similarly, for semi-arid rangeland it has been demonstrated that mounds underneath shrubs have a high infiltrability and lower lying compacted or stony inter-shrub areas have a lower infiltrability. Runoff accumulation further downslope leads to increased water depth, inundating high infiltrability areas, which increases the area-averaged infiltration rate. A model was developed that combines the concepts of water depth-dependent infiltration, partial contributing area under variable rainfall intensity, and the Green-Ampt theory for point scale infiltration. The model was applied to rainfall simulation data and natural rainfall – runoff data from a small sub-watershed (0.4 ha) of the Walnut Gulch Experimental Watershed in the semi-arid US Southwest. Its performance to reproduce observed hydrographs was compared to that of a conventional Green-Ampt model assuming complete inundation sheet flow, but with runon infiltration. Parameters were derived from rainfall simulations and from watershed scale calibration directly from the rainfall – runoff events. The performance of the water depth-dependent model was better than that of the conventional model on the scale of a rainfall simulator plot, but on the scale of a small watershed the performance of both model types was similar. We believe that the proposed model makes some practical and theoretical advancement towards a less scale dependent way of modelling runoff and erosion on the hillslope scale.